The objective of the proposed research is to conduct a comprehensive set of measurements to determine the dielectric properties of malignant, benign, and normal breast tissues over a broad frequency range from 100 MHz to 20 GHz. The outcome of the proposed research will be the baseline data needed to test two hypotheses: a) there exists a systematic and significant contrast in the dielectric properties of normal breast tissue and malignant breast lesions at microwave frequencies, and b) there exists a systematic and significant contrast in the dielectric properties of benign breast lesions and malignant breast lesions at microwave frequencies. The data resulting from these measurements are needed to facilitate the engineering of non- invasive, non-ionizing microwave technology to complement and augment early detection, diagnostic, and treatment technologies presently employed in the detection and management of breast cancer. The long-term objective of our research is to improve breast cancer screening through the use of space-time microwave imaging as a complement to X-ray mammography. The data obtained from the proposed study will permit subsequent development of a practical microwave breast imaging system for clinical trials. The specific aims are: 1) To rigorously calibrate and test coaxial needle probes that will be used with a microwave vector network analyzer to obtain the dielectric permittivities and conductivities of freshly excised breast tissue samples for frequencies up to 20 GHz. 2) To measure at physiologic temperatures and moisture contents the dielectric properties of approximately 500 breast biopsy specimens and 60 breast reduction specimens. 3) To correlate the measured dielectric properties data with the complete surgical pathology reports for every breast biopsy and breast reduction specimen; the database will include fatty and fibroglandular normal breast tissue, benign tumors and proliferative lesions, and infiltrating and in situ carcinomas. 4) To conduct bio-electromagnetic dispersion analyses of the measured dielectric properties data and to assess the biophysical mechanisms for the observed contrasts (or lack thereof). 5) To conduct statistical analyses of the measured data to test the two tissue-dielectric-contrast hypotheses. This research will provide a scientific foundation for all future microwave technology development related to the breast in the 0.1 to 20 GHz range, whether for tumor detection, monitoring, or treatment.